The present invention relates to a module for monitoring the print quality of a dot printer, which monitor is to be fixed to a carriage for moving a printing device of the printer, said module comprising:
an illumination system for supplying a monitoring radiation beam to illuminate an area of a printing medium;
an imaging system for collecting monitoring radiation reflected by the illuminated area and forming an image in an image plane, and
a radiation-sensitive detection system for converting said image into electric signals representing quality parameters of the print.
The invention also relates to a dot printer provided with such a monitoring module.
A dot printer is to be understood to be any kind of apparatus for writing an image on a recording medium whereby the medium produces marks, in the form of dots when exposed, for example, to energies above a threshold level. A number of such dots jointly constitutes a character, a sign, a graphic representation, etc. Well-known dot printers are laser printers, inkjet printers and thermal printers.
U.S. Pat. No. 5,633,672 discloses such an apparatus which comprises a communication channel for carrying an information signal representing the image to be written, a writing assembly, or writing device, coupled to said channel and modulated by the information signal for making marks, representing the image, on a printing medium in response to the signal, a device for producing a relative movement in one direction between the printing medium and said writing assembly to establish a pattern of marks in the direction of the relative movement.
The apparatus described in U.S. Pat. No. 5,633,672 further comprises a calibration instrument that reads the pattern of marks and adjusts the parameters of the apparatus in real time to calibrate the writing device and, when necessary, modify the characteristics of the marks and their pattern. The calibration instrument can be fixed to the drive, or carriage, for the writing device so that it is moved simultaneously with this device. By using this built-in calibration instrument, it is no longer necessary to remove printed images from the apparatus and analyze them off-line for the characterization of the marks and their pattern, which is a cumbersome and time-consuming process. The controlled writing parameters are the size, the relative position and the density of the dots. In the printer described in U.S. Pat. No. 5,633,672, the calibration instrument comprises an electronic (CCD) camera having an optical axis, and the marks are illuminated co-axially along the same axis. The camera receives. a two- dimensional image of the recorded dots and their pattern, for example, if the recording medium carrier is a drum at each rotation of this drum, so that all dots of a print line, or of a number of print lines if the writing device is a multi-channel device, are taken in simultaneously. The camera reads the spatial and density information from the pattern.
It is an object of the present invention to provide a monitoring module of the kind described in the preamble, which module allows measurements on a dot-by-dot basis and, moreover, has a simple and compact construction. The module is characterized in that the detection system comprises at least a linear array of discrete detector elements, and in that means are provided for imaging, in the plane of the detector elements, both a medium area of a first kind, comprising a number of printed dots, and a medium area of a second kind, comprising one printed dot.
By means of this module it is not only possible to image a portion of a print line on the detection system and observe it, to monitor the density or gray scale of the print line portion and to detect the edges of the recording medium, or print paper, but it is also possible to image an individual dot on the detection system. The detection system has detector elements which are so small relative to the image of a dot that such an image covers several detector elements, or pixels, so that characteristics of an individual dot, like its size, can be measured.
The monitoring and calibration can be carried out during a test before recording the desired image, or concurrently with image recording.
It is to be noted that EP Patent 0.186.651 discloses a thermal printer wherein individual dot areas are monitored, however during dot formation and not after the dot formation has been completed. The printer described in EP Patent 0.186.651 is used to carry out a special method of thermal printing. First, initial energy is applied to selected areas of the recording medium to form, in each area, a dot having an initial size which is smaller than necessary to achieve the desired density in the selected area. This area is monitored against a reflective background, in the form of a reflective layer on the rear side of the transparent record medium, by illuminating the area and capturing radiation reflected by the area by a single detector element. This element supplies a signal which is indicative of the density of the area. This signal is compared with a desired image signal and the result of the comparison is used to regulate further supply of thermal energy to progressively increase the dot size in the area until a predetermined value of density is achieved. In the thermal printer described in EP Patent 0.186.651, a dot is not monitored by a number of detector elements and no image of a portion of a print line is formed on the detection system.
A first-embodiment of the module is characterized in that said means for imaging comprises an actuator for positioning the imaging system in either one of two fixed positions along the axis of said imaging system.
In this embodiment, the same lens element(s) of the imaging system is (are) used to image both an individual dot and a print line portion on the detection system. When the imaging system occupies a first one of said positions, a dot is sharply imaged on the detection system and when the imaging system occupies the second position, a portion of the print line is imaged on the detection system. The detection system then receives either an image of an individual dot or an image of the print line portion.
It is also possible to image the individual dot and the print line portion simultaneously on the same detection system. This can be realized with an embodiment of the module which is characterized in that said means for imaging comprises a first and a second sub-lens system of the imaging system, the first sub-lens system having such a power and position that it image a medium area of the first kind on a first region of the detection system, and the second sub-lens system having such a power and position that it image a medium area of the second kind on a second region of the detection system.
Each sub-lens system can now be optimally adapted to the object it has to image and no movement of the imaging system is required. The images of the individual dot and of the print line portion can then be evaluated simultaneously.
Especially for monitoring an individual dot, it is essential that the recording medium, or print paper, remains in the focal plane of the imaging system or of the sub-lens system which images the dot. In practice, it has been found that, during the printing procedure, the medium, at the position of the module, may wobble over a range of +0.5 mm to xe2x88x920.5 mm relative to the nominal plane of said medium. A lens system with which a dot can be measured at an accuracy of +10 xcexcm and fits in the small module, i.e. having a sufficiently small object-to-image distance, has a depth of focus which is smaller than said wobble range. An embodiment of the module which provides a solution to the important problem of the wobbling medium is characterized in that it is provided with an auto-focus system comprising a focus error detection system coupled to a focus correcting actuator.
The auto-focus system ensures that optics the medium is in focus in the field of view of the imaging.
The embodiment of the module wherein the selection between imaging an individual dot or imaging a print line portion is made by moving the imaging system may be further characterized in that the focus-correcting actuator is an actuator for fine-positioning the imaging along its axis.
In this embodiment, also a signal from the focus error detection system may be supplied to the actuator for selecting the imaging mode, so that this actuator performs two functions: selecting the imaging mode, i.e. imaging an individual dot or imaging a print line portion, and tuning the focus.
Alternatively, the module may be further characterized in that the focus-correcting actuator is an actuator for positioning a holder, accommodating all optical elements of the module, relative to the medium in a direction perpendicular to the medium.
The holder actuator can be used not only with the module wherein the imaging system comprises two fixed sub-lens systems, but also in the module with an undivided, movable, imaging system, although focus tuning by the lens actuator is preferred for the latter module.
The module with a holder actuator may be further characterized in that the actuator is constituted by a motor forming part of the module and having a cam wheel which is sustained by a ridge on a back plate, which plate is to be mounted on the carriage.
Upon rotation of the motor, the motor together with the holder is displaced axially. This actuator construction becomes possible because the module is compact and has a low mass.
Several methods and devices may be used for detecting the focus error. The different embodiments of the module are preferably characterized in that the focus error detection system comprises a portion of the radiation-sensitive system and an electronic processing circuit which is coupled to said portion to process and analyze the signals from the detector elements of said portion in order to determine for which position of the axially movable element the image of a feature of the medium has maximum contrast and/or minimum size.
The module may be used in a monochrome printer, but provides an even greater advantage when used in a colour printer because it then enables the colour of an individual dot to be controlled. The module for this purpose is characterized in that the radiation-sensitive detection system comprises a colour beam splitter for splitting radiation incident thereon into radiation of different colours and further comprises a corresponding number of sets of detector elements, each set being intended to receive radiation of one of said different colours radiation""s.
An alternative embodiment of the module for the same purpose is characterized in that a separate colour filter is arranged in front of each detector element of the radiation-sensitive detection system, the colour filters for neighboring detector elements transmitting radiation of different colours.
No separate colour separating element is needed in this embodiment of the module.
If only the presence of the different print colour component: cyan, magenta, yellow and black in the print needs to be controlled, i.e. if it is only to be checked whether the nozzles for the different ink colours indeed eject ink, the radiation source of the illumination system needs to emit radiation of one wavelength band, provided that radiation of this wavelength is sufficiently absorbed by the printed colour dots. It has been found that a light-emitting diode (LED) having a central wavelength of 430 nm is suitable for this purpose. If not only the presence of print colour components, but also the colour composition of printed dots, i.e. the relative amount of print colour components, is to be controlled, a radiation source should be used that supplies radiation comprising colour components which are absorbed most by the cyan, magenta, yellow and black components of the dot. Such a source may be a single source, for example, a small lamp which emits white light.
Preferably, however, the module is further characterized in that the illumination system comprises at least one set of different light-emitting diodes each emitting radiation of a different colour.
Light-emitting diodes, or LEDs have proved to be reliable radiation sources which may be small and cheap and are available for different colours. In practice, the radiation source of the module will comprise a set of three different LEDs emitting the primary colours red, green and blue. Under circumstances, for example, when a higher intensity for the monitoring radiation is required and sufficient room is available, two or more sets of such LEDs may be used.
The LEDs can be switched on simultaneously and the different print colour components are then detected simultaneously. It is also possible to switch on the LEDs successively and to detect the print colour components successively. In the latter case the same detector elements can be used to detect the different print colour components, so that fewer detector elements are needed than in the case where these colour components are detected simultaneously.
If the solid angle of the radiation emitted by the radiation source is not too large, the radiation source may directly illuminate the printing medium. For example, the LEDs may be provided with lens elements which constrict the emitted radiation.
Preferably, however, the module is characterized in that the illumination system comprises a convergent lens system.
Such a lens system concentrates a maximum amount of the source radiation on a limited area of the printing medium so that the source radiation is used efficiently.
The module is preferably further characterized in that the illumination system comprises a cylindrical lens to form an elongated illumination spot on the medium, the longitudinal direction of the spot being parallel to the direction of a print line.
Such a shape and orientation of the illumination spot fits best for the purpose of the module.
The invention also relates to a dot printer for writing an image on a recording medium by exposing the medium on a dot-by-dot basis and comprising:
a communication channel for carrying an information signal representing the image to be printed;
at least one writing device coupled to the communication channel and modulated by the information signal for producing dots on the recording medium, together composing the image;
a carriage for carrying the writing device for producing a relative movement between the recording medium and the writing device, and
a calibration instrument, fixed to the carriage, to read the printed dots and adjust printer settings. This dot printer is characterized in that the calibration instrument comprises a module as described hereinbefore.
The invention further relates to a method of printing information in the form of dots on a printing medium, comprising the steps of:
supplying the information to be printed to a dot printer;
printing a test pattern on the medium;
optically detecting the test pattern;
comparing the test pattern with a reference;
adapting printer settings if a deviation between the reference and the test pattern is detected, and
printing the information supplied to the printer.
This method is characterized in that the step of detecting the test pattern comprises detecting the gray scale of a test print line and detecting the size of at least one individual test dot.
This method ensures a high print quality, also with consumer printers.
This method is preferably characterized in that the step of detecting the test pattern also comprises detecting the colour of a test dot.
The method may be further characterized in that the step of printing a test pattern comprises printing a first portion of the information to be printed.
If dots and print line portions of the print image itself are used for testing, the time required for the calibration process is minimum.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiments described hereinafter.